An inkjet recording method of flying an ink on a recording medium to form a recording dot, thereby performing printing, is attracting attention as a non-impact recording method assured of easy colorization and capable of direct recording on plain paper, and various printers employing this system are put into practice. The inkjet recording method includes an on-demand (on-demand ejection) system and a continuous (continuous ejection) system and these are described, for example, in publications such as Takeshi Agui, et al. (Real Color Hard Copy, Sangyo Tosho (1993)), Shin Ohno (Non-impact Printing—Technology and Material—, CMC (1986)) and Takeshi Amari (Inkjet Printers—Technology and Material—, CMC (1998)).
Furthermore, there are known recording systems called an electrostatic system (Sweet type and Hertz type) for the continuous type, and a piezoelectric system, a shear mode piezoelectric system and a thermal inkjet system for the on-demand type. With respect to the on-demand type inkjet recording method, one known method is a system called an electrostatic acceleration-type inkjet or slit-jet system and this system is described, for example, Susumu Ichinose and Yuji Ohba (IEICE Transaction, J66-C, No. 1, page 47 (1983)) and Tadayoshi Ohno and Mamoru Mizuguchi (The Journal of the Institute of Image Electronics Engineers of Japan, Vol. 10, No. 3, page 157 (1981)). In this system, a voltage is applied to a plurality of recording electrodes disposed to oppose a recording medium and also applied to a counter electrode provided behind the recording medium and due to potential difference generated between both electrodes, an electrostatic force is caused to act on the ink supplied on the recording electrode, as a result, the ink is flown onto the recording medium. Specific embodiments thereof are disclosed, for example, in JP-A-56-170, JP-A-56-4467 and JP-A-57-151374. These are a system in which a long and narrow slit-like ink ejection port having many recording electrodes in the inner wall is used in place of the nozzles in a conventional inkjet head and in which an ink is supplied to the slit-like ink chamber and a high voltage is selectively applied to these electrodes, whereby the ink in the vicinity of the electrode is jetted to the recording paper closely opposing the slit-like head.
Accordingly, clogging of ink less occurs, reduction in the production cost is promised by virtue of the simple head structure, and this method is effective also for realizing a so-called long line head having a length long enough to cover a wide range of a recording medium in the cross direction.
One example of the drop-on-demand type full-color recording head constituted by such an electrostatic acceleration-type inkjet recording system is disclosed, for example, in JP-B-60-59569 and IEICE Transaction, J68-C, No. 2, pp. 93-100 (1985).
However, the oil-based ink used therein has a small surface tension as compared with an aqueous ink commonly used in other inkjet systems and therefore, this ink exhibits very high permeability into the recording paper. Particularly, in the case of printing a letter or an image on plain paper, reduction in the printing density, blurring or strike-through is liable to occur.
An electrostatic system of coloring material concentration-ejection type using no slit-like recording head is disclosed in JP-A-9-193389 and JP-A-10-138493. In this system, a plurality of electrodes for causing an electrostatic force to act on a coloring material component in the ink each is constituted by a control electrode substrate and a convex ink guide, the control electrode substrate comprising an insulating plate having formed therein a through-hole and a control electrode formed correspondingly to the through-hole, and the convex ink guide being disposed nearly in the center position of the through-hole, an ink is carried on the surface of the convex ink guide and transferred to an ink droplet-flying position by a surface tension, and a prescribed voltage is applied to the control electrode to fly an ink droplet on a recording medium, thereby performing recording.
This electrostatic inkjet recording system of coloring material concentration-ejection type is a system of concentrating coloring material particles to an ejection port by electrophoresis and flying an ink droplet in the state of the coloring material being concentrated to a high concentration. In this way, unlike the above-described system of flying an ink droplet still containing a large amount of a liquid component to allow for uniform dispersion of the ink-constituting components, the ink droplet is ejected in the state that the coloring material particles are aggregated and the liquid component content is small, and therefore, the above-described problems are overcome. In addition, a pigment is used as the coloring material, so that advantageous results can be obtained with respect to water resistance and light fastness of the printed image as compared with a conventional inkjet head using a dye.
For obtaining high printing density and good printing property free from blurring or strike-through in such an electrostatic inkjet system of coloring material concentration-ejection type, it is first required that the ink has a sufficiently large volume resistivity. By satisfying this requirement, an electric field formed by the recording electrode and the counter electrode and applied to the ink can be made to reach the color material particles. If the volume resistivity of the ink is low, the ink is subjected to charge injection by a voltage applied from the recording electrode and is electrically charged, as a result, the ink still containing a large amount of a liquid component is more likely to be ejected due to an electrostatic repulsive force. Secondly, since the coloring material particles need to be concentrated to the ejection port at a sufficiently high speed by electrophoresis, it is required that the coloring material particle is electrically charged to a satisfactory level, that is, the coloring material particle has a positive or negative high particle electric conductivity.
In recent years, with increased demand for high speed processing of printer and high image quality, the coloring material concentration-ejection type ink head is also required to develop a technique of printing a highly refined image at a high speed by ejecting a fine ink droplet comprising a coloring material concentrated to a high concentration in a stable and high-speed manner for a long period of time.
Such printing performance has been confirmed to greatly depend on the physical properties of ink and in order to obtain satisfactory printing performance, as described above, the ink must be produced by imparting a high particle electric conductivity of 100 pS/cm or more to the coloring material particle while maintaining a high volume resistivity of preferably 108 Ω·cm or more. If the particle electric conductivity of the coloring material particle is less than 100 pS/cm, the coloring material particles cannot be transferred to the ejection port, that is, the distal end of the ejection electrode, at a high speed by electrophoresis and the supply of coloring material particles becomes insufficient, as a result, the aggregating property of the coloring material particles is worsened and the ejection response frequency decreases.
Furthermore, the repulsive force between the ejection electrode surface and the coloring material particle is weak and therefore, the coloring material particles sometimes adhere and deposit on the ejection electrode, giving rise to failure in stable ejection. From these reasons, there arises a problem that a sufficiently high printing density cannot be obtained and printing cannot be stably performed at a high speed.
The coloring material particle preferably comprises a coloring material such as pigment and a resin. The resin for covering the coloring material is generally desired to have properties such as (1) to form a coloring material mixture by thoroughly covering the pigment surface and keep an appropriate fluidity by the effect of heat or the like, (2) to sufficiently disperse the coloring material particles by covering the coloring material, (3) to be transparent as much as possible, and (4) to be fixed on a recording medium by fixing and thereby impart a sufficiently high scratch resistance. In the case of the electrostatic inkjet ink described above, it is additionally desired (5) to impart a positive or negative high particle electric conductivity to the coloring material particle. The coloring material particle comprising a coloring material covered with a resin can be formed by covering a coloring material with a resin to form a colored mixture, and dispersing the colored mixture in a non-aqueous solvent. However, the ordinary dispersion technique actually allows for production of a coarse particulate coloring material particle or a fine particulate coloring material particle of 0.2 μm or less and can hardly provide a dispersion of coloring material particles having a uniform particle diameter with an average particle diameter of approximately from 0.3 to 4 μm, in which the number of microfine coloring material particles of 0.2 μm or less is small.